Membrane filter arrangements are well known and are used, for example, to separate solid substances from fluids. These substances may, for example, be bacteria, human and/or cells. The membrane filter arrangement permits, by filtration, to enrich the fluid medium with high molecular substances. A system and an arrangement for such a filter is described in the referenced publication, German Patent Disclosure Document DE-OS 38 31 771, Schmeisser et al, assigned to the assignee of the present application.
Filter structures which can carry out such filtrations, as known, utilize two housing plates between which a filter membrane or a filter package is clamped. A circumferential seal seals the filter membrane and/or filter package or array with respect to the outside. The plates which retain the filter package are clamped together. The plates usually are massive, solid plates of high-quality steel, with a thickness of about 1.5 cm. Bores extend through these housing plates in order to be able to supply a filtrate to the membrane filter as well as a concentrate, and to remove filtrate and concentrate therefrom. The connections between the outside and the filter structure usually is by means of a connecting stub which has a connection flange. The stubs are welded to the outside of the housing plates. The connecting flange has a sealing surface which is in cooperate engagement with the sealing surface of a further coupling element for the outside connection, for example a hose. A sealing element as such, for example an O ring, can be located between the connecting stub and the coupling element. The connection, which is a type of flange connection, is retain in place by clamps which press the sealing surfaces as well as the sealing element against each other.
It has been found in operation that the welds on which the sealing stub is welded to the housing plate are subject to defects. Fissures, hairline cracks and the like may occur, for example upon bending or twisting loading placed against the connecting stubs and/or coupling elements. Welding stubs to the plates is expensive and requires meticulous quality control in order to avoid leaks in their construction. Even hairline fissures which will not result in actual fluid leakage may lead to contamination of the environment by molecular substances or substrate escaping through the hairline fissures or cracks; further, it is equally possible that some such leakage points may contaminate the filtrate and/or the concentrate, for example by ingress of external bacteria, which are in the ambient environment.
Fissures or hairline cracks may occur not only due to mechanical stresses and loading, but also by change of the chemical or crystallographic characteristics of the plate and/or connecting stub in the region of the weld, resulting in increased susceptibility to corrosion in the region of the connection.